The present invention is a method and apparatus for enhancing the collection of weakly scattered electromagnetic radiation. More particularly the present invention is a device for intensifying and collecting the scattered radiation which is generated Dy laser-induced Raman scattering. The present invention may be used in conjunction with conventional spectrometric devices to perform molecular gas analysis in a variety of applications.
The designers of detection and monitoring instruments face numerous challenges in detecting the presence and concentration of both known and unknown substances. Many industrial sites can pose serious health hazards to workers due to the concentration of harmful fumes that are emitted by open tanks of cleaners or solvents, plating bath, or paint spraying equipment. One of the greatest perils posed by underground mining operations is the unexpected accumulation of potentially lethal gases. During a surgical operation, patients are anesthetized through the careful administration of gases such as nitrous oxide. The supply of these anesthetics must be regulated with great precision. In addition, the gases expelled in the breath need to be monitored continuously to determine the condition of the patient. Insuring the safety of persons situated in these diverse environments can require the constant monitoring of the substances in the air in which they breathe.
One instrument that is currently employed to detect the presence and concentration of various gases utilizes the physical phenomenon called Raman scattering. The instrument includes a laser which is directed toward a sample of gas contained in a chamber. The laser produces an intense stimulating beam of substantially monochromatic electromagnetic radiation, a small fraction of which is Raman scattered by the constituent gas molecules. This weakly scattered electromagnetic radiation exhibits a change or shift in frequency from that of the stimulating radiation. This change depends on the type of constituent gas which scattered the stimulating radiation, each gas generally having unique spectrum of frequency shifts. By measuring and interpreting the frequencies and intensities of the scattered radiation, the presence and concentration of the various constituent gasses in the sample can be deduced.
At the center of this system is the Raman scattering phenomenon which produces scattered radiation of a very weak intensity. There has been a long felt need to improve the efficiency and lower the cost of Raman scattering based instruments, but progress has been hindered by the weak intensity of the scattered radiation. Efforts to increase the intensity of the scattered radiation that is detected depends upon improving one or more of the following five factors:
1. the Raman molecular scattering cross-section; PA1 2. the number density of the molecules distributed along the path of the stimulating radiation; PA1 3. the intensity of the beam of stimulating radiation which scatters off the contained gasses; PA1 4. the size of the solid angle in which the scattered radiation is collected; PA1 5. the path length over which the beam interacts with the gas molecules;